Hydrogen chloride (HCl) or hydrochloric acid is a reaction by-product of many manufacturing processes which use chlorine. For example, chlorine is used to manufacture polyvinylchloride, isocyanates, and chlorinated hydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as a by-product of these processes. Because supply so exceeds demand, hydrogen chloride or the acid produced often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible. Discharge of the acid or chloride ions into waste water streams is environmentally unsound. Recovery and feedback of the chlorine to the manufacturing process is the most desirable route for handling the HCl by-product. A number of commercial processes have been developed to convert HCl into usable chlorine gas. See e.g., F. R. Minz, "HCl-Electrolysis-Technology for Recycling Chlorine", Bayer AG, Conference on Electrochemical Processing, Innovation & Progress, Glasgow, Scotland, UK, Apr. 21-Apr. 23, 1993.
Currently, thermal catalytic oxidation processes exist for converting anhydrous HCl and aqueous HCl into chlorine. Commercial processes, known as the "Shell-Chlor", the "Kel-Chlor" and the "MT-Chlor" processes, are based on the Deacon reaction. The original Deacon reaction as developed in the 1870's made use of a fluidized bed containing a copper chloride salt which acted as the catalyst. The Deacon reaction is generally expressed as follows: ##STR1##
where the following catalysts may be used, depending on the reaction or process in which equation (1) is used.
Reaction Catalyst or Process Cu Deacon Cu, Rare Earth, Alkali Shell-Chlor NO.sub.2, NOHSO.sub.4 Kel-Chlor Cr.sub.m O.sub.n MT-Chlor
The commercial improvements to the Deacon reaction have used other catalysts in addition to or in place of the copper used in the Deacon reaction, such as rare earth compounds, various forms of nitrogen oxide, and chromium oxide, in order to improve the rate of conversion, to reduce the energy input and to reduce the corrosive effects on the processing equipment produced by harsh chemical reaction conditions. However, in general these thermal catalytic oxidation processes are complicated because they require separating the different reaction components in order to achieve product purity. They also involve the production of highly corrosive intermediates, which necessitates expensive construction materials for the reaction systems. Moreover, these thermal catalytic oxidation processes are operated at elevated temperatures of 250.degree. C. and above.
Electrochemical processes exist for converting aqueous HCl to chlorine gas by passage of direct electrical current through the solution. The current electrochemical commercial process is known as the Uhde process. In the Uhde process, aqueous HCl solution of approximately 22 wt % is fed at 65.degree. C. to 80.degree. C. to both compartments of an electrochemical cell, where exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration to 17 wt % with the production of chlorine gas and hydrogen gas. A polymeric separator divides the two compartments. The process requires recycling of dilute (17 wt %) HCl solution produced during the electrolysis step and regenerating an HCl solution of 22 wt % for feed to the electrochemical cell. The overall reaction of the Uhde process is expressed by the equation ##STR2##
As is apparent from equation (2), the chlorine gas produced by the Uhde process is wet, usually containing about 1 wt % to 2 wt % water. This wet chlorine gas must then be further processed to produce a dry, usable gas. If the concentration of HCl in the water becomes too low, it is possible for oxygen to be generated from the water present in the Uhde process. This possible side reaction of the Uhde process due to the presence of water, is expressed by the equation: EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e.sup.- (3)
Further, the presence of water in the Uhde system limits the current densities at which the cells can perform to less than 500 amps/ft.sup.2, because of this side reaction. The result is reduced electrical efficiency and corrosion of the cell components due to the oxygen generated.
Another electrochemical process for processing aqueous HCl has been described in U.S. Pat. No. 4,311,568 to Balko. Balko employs an electrolytic cell having a solid polymer electrolyte membrane. Hydrogen chloride, in the form of hydrogen ions and chloride ions in aqueous solution, is introduced into an electrolytic cell. The solid polymer electrolyte membrane is bonded to the anode to permit transport from the anode surface into the membrane. In Balko, controlling and minimizing the oxygen evolution side reaction is an important consideration. Evolution of oxygen decreases cell efficiency and leads to rapid corrosion of components of the cell. The design and configuration of the anode pore size and electrode thickness employed by Balko maximizes transport of the chloride ions. This results in effective chlorine evolution while minimizing the evolution of oxygen, since oxygen evolution tends to increase under conditions of chloride ion depletion near the anode surface. In Balko, although oxygen evolution may be minimized, it is not eliminated. As can be seen from FIGS. 3 to 5 of Balko, as the overall current density is increased, the rate of oxygen evolution increases, as evidenced by the increase in the concentration of oxygen found in the chlorine produced. Balko can run at higher current densities, but is limited by the deleterious effects of oxygen evolution. If the Balko cell were to be run at high current densities, the anode would be destroyed.
In general, the rate of an electrochemical process is characterized by its current density. In many instances, a number of electrochemical reactions may occur simultaneously. When this is true, the electrical driving force for electrochemical reactions is such that it results in an appreciable current density for more than one electrochemical reaction. For these situations, the reported or measured current density is a result of the current from more than one electrochemical reaction. This is the case for the electrochemical oxidation of aqueous hydrogen chloride. The oxidation of the chloride ions is the primary reaction. However, the water present in the aqueous hydrogen chloride is oxidized to evolve oxygen as expressed in equation (3). This is not a desirable reaction. The current efficiency allows one to describe quantitatively the relative contribution of the current from multiple sources. For example, if at the anode or cathode multiple reactions occur, then the current efficiency can be expressed as: ##EQU1##
where .eta..sub.j is the current efficiency of reaction j, and where there are NR number of reactions occurring.
For the example of an aqueous solution of HCl and an anode, the general expression above is: ##EQU2##
In the specific case of hydrogen chloride in an aqueous solution, oxidation of chloride is the primary reaction, and oxygen evolution is the secondary reaction. In this case, the current density is the sum of the two anodic reactions. Since .eta..sub.O.sub..sub.2 is not zero, the current efficiency for chloride oxidation is less than unity, as expressed in equations (7) and (8) below. Whenever one is concerned with the oxidation of chloride from an aqueous solution, then the current efficiency for oxygen evolution is not zero and has a deleterious effect upon the yield and production of chlorine. EQU .eta..sub.O.sub..sub.2 .noteq.0 (7) EQU .eta..sub.Cl.sub..sub.2 =1.0-.eta..sub.O2 . . . i.sub.C.sub..sub.l .sub..sub.2 =.eta..sub.C.sub..sub.l .sub..sub.2 .times.i.sub.reported (8)
Furthermore, electrolytic processing of aqueous HCl can be mass-transfer limited. Mass-transfer of species is very much influenced by the concentration of the species as well as the rate of diffusion. The diffusion coefficient and the concentration of species to be transported are important factors which affect the rate of mass transport. In an aqueous solution, such as that used in Balko, the diffusion coefficient of a species is approximately equal to 10.sup.-5 cm.sup.2 /sec. In a gas, the diffusion coefficient is dramatically higher, with values approximately equal to 10.sup.-2 cm.sup.2 /sec. In normal industrial practice for electrolyzing aqueous hydrogen chloride, the practical concentration of hydrogen chloride or chloride ion is approximately equal to 17 wt % to 22 wt %, whereas the concentration of hydrogen chloride is 100% in a gas of anhydrous hydrogen chloride. Above 22 wt %, conductance drops, and the power penalty begins to climb. Below 17 wt %, oxygen can be evolved from water, per the side reaction of equation (3), corroding the cell components, reducing the electrical efficiency, and contaminating the chlorine.
A variety of operating parameters have been investigated, including various anode/cathode catalyst pairs, catalyst loadings, membrane types, reactant flow rates, pressures, cell temperature, and cathode humidification levels. One of the main challenges has been keeping the membrane and anode catalyst layers adequately hydrated in the reactors, so that they retain high protonic conductivity and electrode kinetic activity. This is a problem particularly with PEM-based electrochemical reactors because ideally only the cathode side of the cell is humidified, whereas the anode feed is anhydrous hydrogen chloride.
U.S. Pat. No. 5,411,641 uses a cathode current collector with channels to hydrate the membrane and increase the efficiency of proton transport. This system however only provides for partial hydration which limits the overall efficiency of the system.
As higher currents are drawn from the cell, the effect of the electroosmotic drag of water from the anode to the cathode outweighs the diffussional transport of water from the cathode to the anode, eventually depleting the membrane water content near the anode. Water depletion in the membrane increases the membrane resistance, and limits the current. This effect is less apparent in the water saturated cathode embodiment, however, at current densities higher than 1A/cm.sup.2, the effect of electro-osmotic drag becomes apparent. See, Electrochemical Conversion of Anhydrous Hydrogen Chloride to Chlorine in a Proton Exchange Membrane Reactor. Tatapudi, Electrochemical Soc. Proc. vol. 95-12 p. 142-151.
Another disadvantage of electrochemical conversion of HCl using proton exchange membrane electrochemical reactors, is the poisoning of the cathode catalyst with Cl.sup.- ions resulting from HCl crossover. A system that reduces the level of HCl crossover would be desirable.
Therefore, there is a need for an improved method and apparatus for the electrochemical conversion of anhydrous hydrogon halide gas to essentially dry halogen gas. More particularly, there is a need for a proton conducting membrane that supports high current densities without drying out even when the anode chamber is dry. It would be desirable if the proton conducting ability of the membrane would remain high even if the fluids provided to the cathode chamber are dry, such as when an oxygen depolarized cathode is used. It would be further desirable if the membrane could be used to reduce halide ion crossover from the anode to the cathode.